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blktrace: ensure our debugfs dir exists 

We make an assumption that a debugfs directory exists, but since
this can fail ensure it exists before allowing blktrace setup to
complete. Otherwise we end up stuffing blktrace files on the debugfs
root directory. In the worst case scenario this *in theory* can create
an eventual panic *iff* in the future a similarly named file is created
prior on the debugfs root directory. This theoretical crash can happen
due to a recursive removal followed by a specific dentry removal.

This doesn't fix any known crash, however I have seen the files
go into the main debugfs root directory in cases where the debugfs
directory was not created due to other internal bugs with blktrace
now fixed.

blktrace is also completely useless without this directory, so
this ensures to userspace we only setup blktrace if the kernel
can stuff files where they are supposed to go into.

debugfs directory creations typically aren't checked for, and we have
maintainers doing sweep removals of these checks, but since we need this
check to ensure proper userspace blktrace functionality we make sure
to annotate the justification for the check.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Luis Chamberlain 2020-06-19 20:47:29 +00:00 committed by Jens Axboe
parent bad8e64fb1
commit b431ef837e
1 changed files with 12 additions and 0 deletions
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12
kernel/trace/blktrace.c
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@ -538,6 +538,18 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
#endif
bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
/*
* As blktrace relies on debugfs for its interface the debugfs directory
* is required, contrary to the usual mantra of not checking for debugfs
* files or directories.
*/
if (IS_ERR_OR_NULL(dir)) {
pr_warn("debugfs_dir not present for %s so skipping\n",
buts->name);
ret = -ENOENT;
goto err;
}
bt->dev = dev;
atomic_set(&bt->dropped, 0);
INIT_LIST_HEAD(&bt->running_list);